The number of sensory hair cells in the auditory endorgans of the mammalian ear are not known to change after birth. In contrast, sensory hair cells continue to be added for at least several years (if not throughout the life of the animal) in the otic endorgans that have been studied in elasmobranchs, bony fishes, and amphibians. Since the sensory hair cells of these species are morphologically and functionally similar to those found in birds and mammals, elasmobranchs, bony fishes or amphibians could potentially provide 'model' systems for the experimental investigation of hair cell development, maturation, and cell death. This study will focus on the sensory hair cells of the ears of fishes since earlier work has demonstrated extensive addition of sensory hair cells (average of more than 167 cells per day) for a long time post-embryonically. However, in order to more fully make use of the fish ear as a system for the study of hair cells production we need to generate base-line data on the patterns of development in post-embryonic animals, to compare development to embryonic fish and mammalian cells, and to determine whether cell replacement occurs in adult animals. The specific investigations we will conduct will include: (a) morphological changes in sensory hair cells as they mature in post-embryonic and embryonic animals; (b) the sites on the post-embryonic and embryonic sacculi where new sensory hair cells are added; and (c) determination of hair cell replacement in ears in which hair cells are no longer being added. These studies will ultimately lead to investigations of the anamniotic ear with in vitro studies wehre it will be possible to experimentally manipulate the hair cells environment in order to study aspects of its growth and the effect on growth of such things as ototoxic drugs. Investigations will involve labeling proliferating tissue with 3H thymidine and then doing autoradiography to determine sites of cell addition. Light and transmission electron microscopy will be used for evaluation of tissue. Other investigations will involve use of scanning electron microscopy to determine the numbers of sensory cells in various otic endorgans.